2)
In addition, they didn't have any other impression they would usually have (e.g. it's tough). In other words, they got used to receive plenty of homework.
3)
The meaning is the same. 宿題 can't be misinterpreted as the subject of 追加練習をする (i.e., 宿題が追加練習をする is semantically odd), therefore the readers can recognize 追加練習をする is the contents of 宿題 correctly. Whether という can be omitted or not depends on the head noun.